Interstation communication system



Apil 30,1940.

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Patented Apr; 30, 1940 2.199320 INTERSTATION COMMUNICATION SYSTEM Hubert French, Brooklyn, N. Y. Application July 18, 1939, Serial No. 285,072

13 Claims.

located so close to each other that provision must be made to prevent acoustical feed back from each loudspeaker through itsassociated microphone to'the other station loudspeaker. Heretofore, such provision has been in the nature of manually or electromagnetically actuated means for rendering one transmission line inoperative whenever the other transmission line is operative or has been in the nature of acoustical insula- ;tion between each loudspeaker and the associated microphone.

An object of this invention is the provision in an interstation communication system of electronically controlled means for automatically preventing transmission of signals from one microphone to the loudspeaker connected thereto whenever signals are being transmitted from the other microphone to the loudspeaker connected to it and for preventing transmission of signals of less than a given intensity from either microphone to the loudspeaker at the other station.

Other objects, novel features and advantages of this invention will become apparent from the following specification and accompanying drawings, wherein: r

Fig. l is a circuit diagram of one embodiment of the invention;

Fig. 2 is a circuit diagram of a modified embodiment of the invention, and

Fig. 3 is a circuit diagram of still another embodiment of the invention.

Referring now to Fig. 1 wherein a two-station speaker S are provided at each station in juxtaposition, it being common practiceto have the microphone and speaker in effect a unitary arrangement. The circuit arrangement at the two stations are identical and the same reference characters are used for corresponding elements apparatus is illustrated, a microphone M and a a decoupling resistor Ra, a'resistor R5 and the cathode biasing resistor R9 and a condenser C1 is bridged between said last-named terminal and the cathode of said tube. The cathode of each tube VT1 is connected through said cathode bias- 5 ing resistor R9 to the negative side of a voltage source while the anode of said tube is connected through-the usual plate load resistor R10 to the positive side of said source. Each tube VT1 has a suppressor electrode bridged to thecathode and 10 a screen electrode connected to the positive side of the voltage source in the usual manner through a dropping resistor and by-pass condenser. The output circuit of each tube VT1 is coupled by means of the condenser C2 to the control electrode of a vacuum tube VT2. The cathode of each tube VT2 is connected to the negative side of .the voltage source through the cathode biasing resistor R11.

In the output circuit of each vacuum tube VT2 is contained theprimary winding of a transformer T: which is bridged by a variable voltage divider R1. Each tube VT2 has a suppressor electrode bridged to the cathode and a screen electrode connected to the positive side of said voltage source. The terminalsof the secondary of each transformer T2 are connected to a pair of rectifier anodes of a combination diode and triode vacuum tube We. The midpoint of the secondary of each transformer T2 is connected to the cathode of the associated vacuum tube VT: through a choke coil L1, input line 2 of the opposite' station, the resistor R5 at the opposite station and a common return line connecting the negative sides of. the voltage sources at the 5 two stations. The cathode of each tube VTa is connected to the negative side of the voltage source and the control electrode of the triode section is connected to the. load end of the choke coil L1. Four resistors R2, R3, R4 and R1 are in: 40 terconnected at a common junction and the anode of each tube We is connected through'the resistors R4 and Rate the positive side of the potential source. I v

The slider of each voltage divider R1 is coupled through a condenser C3 to the control electrode of a vacuum tube VT4, such control electrode also being connected through the resistance Re to the anode of tube v'r5. The anode of each tube VT4 is connected through the primary of a transformer T3 to the positive side of the voltage source and the coil of the speaker is connected in series with the secondary of said trans-' former T3. The'tube VT; has' a supmessor electrode bridged to the cathode of said tube and a r screen electrode connected to the positive side of said voltage source. The cathode of each tube VT4 is connected through the cathode biasing resistors R7 and R2 to the negative side of said voltage source. .A by-pass condenser C is provided from the cathode to the negative side of the voltage source in shunt to the cathode biasing resistors and a condenser. C4 is connected in shunt to the resistors R4 and R7.

Whe'n no signals are being transmitted from one station to the other,'the potential on the control electrode of the triode section of each tube VT: is such that plate current flows through said triode section and the voltage drop across resistor R4 biases the control electrode of the associated tube VT4 sufficiently negative with respect to the cathode of such tube to suppress plate current flow in saidtube VT4 therebvinsuring that noise level-of less than a predetermined intensity at either station will not be reproduced in the speaker at the other station.

Upon excitation of either microphone by audible signals of a given intensity, such signals will be reproduced in the speaker at the opposite station. The audible signals are translated into electrical impulses by the microphone and are impressed upon the transformer T1 of the receiving station. The voltage thus induced in the secondary of the receiving station transformer 30. T1 is amplified by vacuum tubes VT1, VT: in the usual manner and a portion of .the output of the tube VT2 is impressed upon the control electrode of the receiving station tube VT-i. Simultaneously, a portion of the output of said tube VT: is rectified by the diode section of the receiving station tube VT: and causes current flow through the transmitting station input line 2, the transmitting station resistor R5 and back through the common return line to the cathode of said diode section. The voltage drop across the transmitting station resistor R5 biases the control electrode of the receiving station tube VT: suificiently negative relative to the cathode to suppress current flow in the tube VT3. As a result of the suppression of the plate current flow in tube VT3, the voltage drop across resistor R4 disappears, leaving a voltage drop created by the resistor Rq'as the only bias for the control electrode of the receiving station tube VT4. Thereupon, the plate current flows in said tube VT4 and such tube reproduces signals from the receiving station tube VTz. Such signals are impressed by the output of the tube VT4 on the receiving station speaker S and .the audible signals impressed upon the transmitting station microphone are reproduced by the receiving station speaker. The voltage drop across the transmitting station resistor R5 likewise biases the control electrode of the transmitting station tube V'Ir relative to its cathode sufliciently negative to suppress plate current flow in said tube, thereby preventing operation of the amplifier whereby the transmitting station speaker is prevented from reproducing signals picked up by the receiving station microphone from the same station speaker.

The modification disclosed in Fig. 2 differs from that disclosed in Fig. 1 in that in Fig. 2,

the amplifying and controlling equipment for the' station 2 speaker is located at station I while the amplifying and controllling equipment for station I is located at station 2 with the result that amplified signals are-transmitted from one station to the other. In this modification, the coil of each microphone is series connected with the primary of the transformer T1 at the same station, while the coil of each speaker is series connected with the secondary of the transformer T3 at the opposite station. The operation of Fig. 2 modification, however, is identical with that of the Fig. 1 modification.

In the modification disclosed in Fig. 3, the coil of the microphone at each station is series connected to the primary of the transformer T! at the same station. One terminal of the see- I ondary of each transformer Tris connected to the control electrode of a vacuum tube VT1, while the other terminal of said secondary isconnected to the cathode of said tube through a decoupling resistor R1, resistor R2 and cathode biasing resistor R11, and a condenser C1 is bridged between said last-named terminal and the cathode of said tube. The cathode of each tube VT1 is connected through said cathode biasing resistor R11 to the negative side of a voltage source, while the anode of said tube is connected through the usual plate load resistor R12 to the positive side of said source. Each tube VT1 has a suppressor electrode bridged to the cathode and a screen electrode connected to the positive side of the voltagesource, in the usual manner through a dropping resistance and by-pass condenser. The output circuit of each tube VT1 is couplied by means of the condenser C3 to the control electrode of a vacuum tube VTz. The cathode of each tube VT2 is connected to the negative side of the voltage source through the cathode biasing resistor R13 and by-pass condenser. In the output circuit of each vacuum tube VTz is contained the primary winding of a transformer T2. One terminal of the secondary of each transformer T2 is connected to one of a pair of rectified anodes of a combination diode and triode vacuum tube VT3. The anode of the tube VT1 is coupled through thecondenser C2 to the control electrode of the vacuum tube VT; and the anode of the tube VT; is coupled to said control electrode through the resistor R5. In the output circuit of the vacuum tube VT4 is included the primary of the transformer T3, the secondary of which is connected in series with the secondary of the transformer T3 at the opposite station. Also, the output circuit of each tube VT4 is coupled through the condenser C8 to the control electrode of a vacuum tube VT5, the output circuit of which includes the primary winding of a transformer T4, the secondary of which is connected in series with the coil of the speaker S. The remaining anode plate of the diode section of VT: is coupled through the condenser C to the anode of the same station tube W5. The control electrode of the triode section of each tube VT: is

connected through a resistor R4, resistor Rm, and resistor R14 to the control electrode of vacuum tube VTa. The remaining end of the secondary of each transformer T2 is connected through the resistor R3 bridged by the condenser 04 to the negative side of the voltage source. A condenser C5 bridges the control electrode of the vacuum tube VT3 to the negative side of the voltage source. Four resistors R6, Ra, Re and R9 are interconected at a common junction and the anode of each tube VT: is connected through the resistors Re and R9 to the positive side of the voltage source. The cathode of each vacuum tube VT4 is connected through cathode biasing resistors R1 and Re to the negative side of the A condenser Cs bridges the re-v a eaaao In the modification of Fig. 3, the transmission between stations I and 2 takes place over two conductors only which makes it necessary to generate locally at each station all the biasing voltages which may be necessaryfor the prevention of acoustical feed back between a speaker and -a microphone during transmitting, receiving and silent periods. 'When neither station is transmitting, the control electrodes of tube V'Pe is biased negatively with" respect to its cathode sufliciently to prevent plate-current flow in the tube VT4 Such biasing of the control electrode of the tube VT; is produced by the voltage drop across the resistor R6 in the-output circuit of the triode section of the tube VTa, the control electrode of which is of proper bias with respect to'its cathode as to permit plate current flow.

'When the station. I is a transmitting station, the

audible signals actuate-the station I microphone and their electrical equivalents are transmitted by induction through transformer T1 to the controlelectrode of the tube VT1.v amplified output of the tube;VT1 is impressed through the coupling condenser C: on the grid of VT2 which tube further amplifies the signals.

From the output of the vacuum tube VTa, the signal is induced into the secondary winding of the transformer T2 from whence it is fed to one of the diode anodes in the tube VT3. A voltage of one polarity is developed across the condenser- C4 which negatively biases the control electrode of the triode section of the vacuum tube VT: and also negatively biases the control electrode of the tube V'I's. Such biasing of the control electrodes just referred to is suflicient to prevent the flow of plate current in the triode section of 7 the tube VT: and also to prevent plate current As a result of the interfiow in the tube VTs. ruption of the plate current flow in the triode section of the tube VTa, the voltage drop across the resistor R6 is eliminated and the high negative bias previously existing on the control electrode of tube VT4 and which previously had prevented fiow of plate current in the tube VT4 is removed thereupon permitting such tube to become operative. The remaining signal output of the tube VT1 is impressed through the condenser C2 on the control electrode of the tube VT4 which has now as above described been rendered'operative. The output of the tube VT4 is induced into the secondary winding of the transformer T3 and is transmitted through the lines I and 2 to the receiving station. Theoutput of the tube VT4 is prevented from operating the tube VTs by the application tothe control electrode of the tube VTs of a high negative biasing voltage obtained through the decoupling resistor R10 from resistor R3. From the secondary of the receiving-station transformer T3, the'signal is impressed through the coupling-condenserCe on the control electrode of the tube V'Ts and the output of the tube VTs is impressed upon the receiving station speaker from the 'transformer.T4. In order that the sound output of the receiving station speaker can produce no input to the control electrode of the receiving f station tube VT1, a small portion A part of the I of the output s a1 er the receiving station tube VT; is'fed through the coupling condenser C10 to the diode section-of the; receiving station tube for rectification. The resultin rectified voltage appears across the receiving station resistor R2, the negative side of which is connected the decoupling resistor' R1, thereby negatively biasing the control electrode of the tube VT1 sufliciently to prevent plate current flow in said tube,

. to the control electrode of the tube v'r1 through The vacuum tubes and VT: of Figs. 1 and 2 amplify the signals sufiiciently to provide the voltage necessary properly to bias the controlelectrode of the opposite station tube VTi and'the control electrode of the same'station tube VT: to interrupt plate. current flow in such tubes. The tube VT4 constitutes a normally inoperative repeater which is rendered operative by reduction of its negative bias when the plate current flow is interrupted in the.tub,e VT: and may also act as an amplifier if additional amplification is desired. The action is the same in the system of Fig. 3 except that the negative bias for the tube VT1 is developed at the samestation at which the tube is located.

I claim: a I

' 1. In an interstation communication system having a microphone and a speaker at each station, connections between the microphone at each station and the speaker at an opposite station including a vacuum tube amplifier and a vacuum tube repeater between each amplifier and its associated speaker, a triode electron discharge device for each repeater having a resistor in its output circuit, connections for negatively biasing the control electrode of the associated repeater by the potential drop across said resistor, a rection, connections between the microphone-at each station and the speaker at an opposite station including a vacuum tube amplifier and a vacuum tube repeater between each amplifier and its associated speaker, a triode electron discharge device 'for each repeater having a 'resistor in its output circuit, connections for negatively biasing the control electrode of the associated repeater by the potential drop across said resistor, a rectifier for each amplifier connected to theoutput thereof and having a resistor'in series, and connections for negatively biasing the control electrodes of each amplifier and the triode associated with the opposite amplifier bythe voltage drop across the resistor in series with the rectifier associated with said opposite amplifier.

I 3. In an interstation communication system having a microphone and a speaker at each station, connections between the microphone at each station and the speaker at an opposite station thereof and having a resistor in series, and connections for negatively biasing the control electrodes of each station amplifier and the other Station triode by the resistor in series with the rectifier associated with such amplifier.

4. In an interstation communication system having a microphone and a speaker at each station, connections between the microphone at each station and the opposite station speaker including a vacuum tube amplifier and a vacuum tube repeater at each station with transmission lines leading from each repeater to the opposite stat'ion speaker; a triode vacuum tube at each station having a resistor in its output circuit, connections for negatively biasing the control electrode of the same stationrepeater by the potential drop across said resistor, a rectifier at each station connected to the output of the same station amplifier and having a resistor in series, and connections ior negatively biasing the control electrodes of each amplifier and the opposite station triode by the voltage drop across the resistor in series with the rectifier associated with the opposite station amplifier.

5. In an interstation communication system having a microphone and a speaker at each station, connections between the microphone at each station and the opposite station speaker including a vacuum tube amplifier and a vacuum tube repeater at each station with transmission lines leading from each microphone to the opposite station amplifier, a triode vacuum tube at each station having a resistor in its output circuit, connections for negatively biasing the control electrode of the same station repeater by the potential drop across said resistor, a rectifier at each station connected to the output of the same station amplifier and having a resistor in series, and connections for negatively biasing the control electrode of each amplifier andthe control electrode of the opposite station triode by the voltage drop across the resistor in series with the rectifier associated withthe opposite station amplifier.

6. In an interstation communication system having a microphone and a speaker at each station, connections between the microphone at each station and the speaker at an opposite station includinga vacuum tube amplifier and a-vacuum tube repeater between each amplifier and its associated speaker, a triode electron discharge device for each repeater having a resistor in its output circuit, connections for negatively biasing the control electrode of the associated repeater by the potential drop across said resistor, a rectifier for each amplifier connected to the output thereof and having a resistor in series, and connections for negatively biasing the control electrode of one triode by the potential drop across one of said and having a resistor in series, and connections for negatively biasing the control electrode of each triode by the voltage drop across the resistor in series with the same station rectifier.

8. In an interstation communication system having a microphone and a speaker at each station, connections between the microphone at each station and the opposite station speaker including a vacuum tube amplifier and a vacuum tube repeater at each station with transmission lines leading from each repeater to the opposite station speaker, a triode vacuum tube at each station having a resistor in its output circuit, connections for negatively biasing the control electrode of the same station repeater by the potential drop across said resistor, a rectifier at each station connected to the output of the same station amplifier and having a resistor in series, and connections for negatively biasing the control electrode of each triode by the voltage drop across the resistor in series with the rectifier associated with the same station amplifier.

9. In an interstation communication system having'a' microphone and a speaker at each station, connections between the microphone at each station and the speaker at an opposite station including a vacuum tube amplifier and a normally inoperative vacuum tube repeater between each amplifier and its associated speaker, and means responsive to the output voltage of the amplifierfor rendering said repeater operative.

10. In an interstation communication system having a microphone and a speaker at each station, connections between the microphone at each station and the speaker at an opposite station including a vacuum tube amplifier and a normally inoperative vacuum tube repeater between each amplifier and its associated speaker, and means for rendering operative the same station repeater and rendering the opposite station amplifier inop-,

erative.

11. In combination, an amplifier, a vacuum tube repeater connected to the output of said amplifier, a triode electron discharge device having a resistor in its output circuit, connections for negatively biasing the control electrode of said repeater by the potential drop across said resistor, a rectifier connected to the output of said amplifier and having a resistor in series, and connections for negatively biasing the control electrode of said triode by the potential drop across said last-named resistor.

12. In an interstation communication system having amicrophone and a speaker at each station, connections between the microphone at each station and the opposite station speaker including a vacuum tube amplifier and a first vacuum tube repeater at each station with transmission lines leading between the outputs of said first repeaters and a second repeater at each station coupling the first repeater to the speaker, a triode vacuum tube at each station having a resistor in its output circuit, connections. for negatively biasing the'control electrode of each first repeater by the potential drop across said resistor, means including a rectifier for negatively biasing the control electrode of the same station triode and the control electrode of the same station second repeater by said amplifier output voltage, and means for negatively biasing the control electrode of each station amplifier by the output voltage of the same station second repeater.

13. In combination, an amplifier, a vacuum tube repeater connected to the output of said amplifier, a resistor in the anode-cathode circuit of said repeater, connections for negatively biasing the control electrode or said repeater by the potential drop across said resistor, a triode electron discuit, a rectifier connected to the ontput of said amplifier and having a resistor in series, andconnections for negatively biasing the control electrode of said triode by the potential drop across said iast-named'resistor.

- 4 HUBERT FRENCH. 

